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where pi is the p-value for the ith hypothesis test. When the p-values tend to be small, the test statistic X2 will be large, which suggests that the null hypotheses are not true for every test.

When all the null hypotheses are true, and the pi (or their corresponding test statistics) are independent, X2 has a chi-square distribution with 2kdegrees of freedom, where k is the number of tests being combined. This fact can be used to determine the p-value for X2.

The null distribution of X2 is a chi-square distribution for the following reason. Under the null hypothesis for test i, the p-value pi follows a uniform distribution on the interval [0,1]. The negative natural logarithm of a uniformly distributed value follows an exponential distribution. Scaling a value that follows an exponential distribution by a factor of two yields a quantity that follows a chi-square distribution with two degrees of freedom. Finally, the sum of k independent chi-square values, each with two degrees of freedom, follows a chi-square distribution with 2k degrees of freedom.

In the case that the tests are not independent, the null distribution of X2 is more complicated. Dependence among the pi does not affect the expected value of X2, which continues to be 2k under the null hypothesis. If the covariance matrix of the logepi is known, then it is possible to calculate the variance of X2, and from this a normal approximation could be used to form a p-value for X2. Dependence among statistical tests is generally positive, which means that the p-value of X2 is too small (anti-conservative) if the dependency is not taken into account. Thus, if Fisher's method for independent tests is applied in a dependent setting, and the p-value is not small enough to reject the null hypothesis, then that conclusion will continue to hold even if the dependence is not properly accounted for. However, if positive dependence is not accounted for, and the meta-analysis p-value is found to be small, the evidence for the alternative hypothesis is generally overstated. The approximate false discovery rate, /, reduced for positively correlated tests, may suffice to adjust alpha for useful comparison to an over-small p-value from Fisher's X2.

Fisher's method is typically applied to a collection of independent test statistics, usually from separate studies having the same null hypothesis. The meta-analysis null hypothesis is that all of the separate null hypotheses are true. The meta-analysis alternative hypothesis is that at least one of the separate alternative hypotheses is true.

In some settings, it makes sense to consider the possibility of "heterogeneity," in which the null hypothesis holds in some studies but not in others, or where different alternative hypotheses may hold in different studies. A common reason for the latter form of heterogeneity is that effect sizes may differ among populations. For example, consider a collection of medical studies looking at the risk of a high glucose diet for developing type II diabetes. Due to genetic or environmental factors, the true risk associated with a given level of glucose consumption may be greater in some human populations than in others.

In other settings, the alternative hypothesis is either universally false, or universally true – there is no possibility of it holding in some settings but not in others. For example, consider several experiments designed to test a particular physical law. Any discrepancies among the results from separate studies or experiments must be due to chance, possibly driven by differences in power.

In the case of a meta-analysis using two-sided tests, it is possible to reject the meta-analysis null hypothesis even when the individual studies show strong effects in differing directions. In this case, we are rejecting the hypothesis that the null hypothesis is true in every study, but this does not imply that there is a uniform alternative hypothesis that holds across all studies. Thus, two-sided meta-analysis is particularly sensitive to heterogeneity in the alternative hypotheses. One sided meta-analysis can detect heterogeneity in the effect magnitudes, but focuses on a single, pre-specified effect direction.

A closely related approach to Fisher's method is based on Z-scores rather than p-values. If we let Zi = Φ−1(1−pi), where Φ is the standard normal cumulative distribution function, then

is a Z-score for the overall meta-analysis. This Z-score is appropriate for one-sided right-tailed p-values; minor modifications can be made if two-sided or left-tailed p-values are being analyzed. This method is named for the sociologist Samuel A. Stouffer.

Since Fisher's method is based on the average of −log(pi) values, and the Z-score method is based on the average of the Zi values, the relationship between these two approaches follows from the relationship between z and −log(p) = −log(1−Φ(z)). For the normal distribution, these two values are not perfectly linearly related, but they follow a highly linear relationship over the range of Z-values most often observed, from 1 to 5. As a result, the power of the Z-score method is nearly identical to the power of Fisher's method.

One advantage of the Z-score approach is that it is straightforward to introduce weights. If the ith Z-score is weighted by wi, then the meta-analysis Z-score is

which follows a standard normal distribution under the null hypothesis. While weighted versions of Fisher's statistic can be derived, the null distribution becomes a weighted sum of independent chi-square statistics, which is less convenient to work with.